More specifically the invention refers to a circuit able to realise two power buses symmetric with respect to the power voltage negative, without the need for a central socket in the power supply itself and without the need for feedback on the negative bus.
As is known, in the case of electronic circuits, it is often necessary to reconstruct an alternate voltage starting with a direct one.
The direct voltage at input has to be transformed into two direct voltages symmetric with respect to the central point that joins the positive of one voltage to the negative of the other. This occurs, for instance, in half-bridge inverters that realise the two positive and negative half waves, taking the respective direct voltages at the ends of two buses, symmetric with respect to the central zero and connected to the above-described polarities.
The two buses normally have a higher power voltage than the available source and a power step-up circuit is therefore required which, starting with the lower voltage, is able to generate the two high bus voltages symmetrically with respect to the central zero.
Considering that at output, power draw occurs at the same time as the half wave involved, each of the two buses must be able to provide this power in a fully independent way from the other bus, otherwise there would be a strong rise in the power voltage of one when power is drawn from the other and vice versa.
A typical circuit diagram showing an inverter circuit able to reconstruct the alternate mains voltage starting from the direct voltage at input is shown in FIG. 1.
As can be seen from this figure, the direct power source practically consists of two distinct sources VA and VB connected in series with central zero so the circuit itself can be easily doubled into two single circuits, one for the negative voltage and the other for the positive voltage.
The two circuits are completely symmetrical with each other.
The drawback of this solution however is that the overall voltage which has to be used is double with respect to that necessary in absolute value, apart from its non-optimal use.
In fact, alternate use is normally made of half at a time of the power actually available at the positive and negative output half wave respectively.
The circuit in FIG. 1 comprises a step-up circuit made up of an inductor 3, a switch 6 and a diode 7, as well as a condenser 10, designed to stabilise the voltage of the positive bus V1.
The inductor 3 and the diode 7 are arranged in series respectively, while the switch 6 is in parallel to the condenser 10 and supply source VA.
A similar step-up circuit is provided for the second supply source VB and comprises in this case an inductor 4, a switch 5, a diode 8 and a condenser 9, corresponding respectively to inductor 3, switch 6, diode 7 and condenser 10 of the step-up circuit described above.
The output voltage of the circuit is drawn at the ends of a condenser 16 which is connected at a central node between the condensers 9 and 10.
The step-up circuits described above therefore make it possible to raise the voltage VA to the voltage of the positive bus V1 and the voltage VB to the voltage of the negative bus V2.
The two voltages thus obtained are used by two further switches 11 and 12 which, suitably operated at high frequency (normally above the hearing interval) and with the aid of the re-circulation diodes 13 and 14, connected in parallel respectively to the switches 11 and 12, reconstruct the low-frequency sinusoid (typically 50 or 60 Hz).
An LC filter consists of previously described condenser 16 and an inductor arranged in series to this and this filter eliminates the residual high-frequency component from the output voltage VOUT.
In this configuration, the voltage available at the ends of the condensers 10 and 9 depends on the relation between the “on” and “off” time of the respective switches 6 and 5, as well as obviously on the load applied to their ends.
Considering that, as was said before, the absorption from the two buses alternates following the wave shape of the alternate voltage VOUT obtained by them, the control of the two switches 5 and 6 must be independent and each tied to the value of the corresponding bus voltage.
A further drawback of the circuit of FIG. 1 is the fact of having to pilot the two switches 5 and 6 which are at opposite polarities with respect to the central zero, normally used as negative by the control logic. In fact, while it is relatively easy to pilot the switch 6 which is at positive voltage with respect to the zero and therefore “at the same side” of the control logic, piloting the switch 5 which is at negative voltage with respect to the control logic is a little more complicated.
The circuit of FIG. 1 is simply an example of many circuit solutions of known type, all of which have in common a double direct supply source, connected in series the one to the other, together with relevant electronic circuitry, used to make two high-voltage buses, these too connected in series the one to the other.